In an acoustic piano, such as a grand piano, an action operates in accordance with depression of an associated key, to cause pivotal motion of an associated hammer, and the pivotally moved hammer strikes a string to thereby generate a piano tone. Further, in the acoustic piano, the jack of the action is configured to be released from the hammer before the hammer strikes the string. Before and after the release of the jack from the hammer, the touch weight of the key is increased and then sharply reduced, whereby a so-called let-off effect is obtained. The let-off effect is peculiar to an acoustic piano, and therefore in a keyboard instrument, such as an electronic piano, as well, it is preferable to obtain the let-off effect so as to make the touch feeling of the electronic piano analogous to that of an acoustic piano.
As an electronic piano having a let-off mechanism for imparting the above-mentioned let-off effect, there has been known one disclosed e.g. in Patent Literature 1. The let-off mechanism comprises a protruding member protruding from the lower surface of a key and an elastic member attached to a chassis. In the electronic piano, the protruding member is brought into temporary engagement with the elastic member by key depression, whereby the let-off effect is obtained. Further, on the chassis, there are disposed a first switch and a second switch in a manner opposed to the key. Each of the first and second switches is in the OFF state in a key-released state, and when the key pivotally moving in accordance with key depression reaches a first predetermined position P11 and a second predetermined position P12, the first and second switches are sequentially turned on, respectively, in the mentioned order. As shown in FIG. 12, the first predetermined position P11 corresponds to a key depression depth of the key in a final stage of a let-off, and the second predetermined position P12 corresponds to a key depression depth of the key around termination of the let-off. Further, in the musical tone control system, the tone volume of a musical tone to be output is set based on a key depression velocity calculated based on a time period from the turn-on of the first switch to the turn-on of the second switch, and sounding timing for sounding the musical tone is set based on timing in which the second switch is turned on.
However, in the conventional musical tone control system, each of the first and second switches is configured to detect the key-depressed state of the key after the final stage of the let-off, and therefore if sounding stop timing is set based on timing in which the first or second switch is switched from ON to OFF, sounding of a musical tone is stopped in too early timing, e.g. immediately after the key starts return motion, which causes odd feeling in performance.
Further, as another musical tone control system for an electronic piano having the let-off mechanism, there has been known one disclosed e.g. in Patent Literature 2. This electronic piano has a first switch and a second switch disposed in a manner opposed to an associated hammer. Each of the first and second switches is in the OFF state in a key-released state, and when an associated key pivotally moving in accordance with key depression reaches a first predetermined position P11′ and a second predetermined position P12′, the first and second switches are turned on, respectively. As shown in FIG. 13, the first predetermined position P11′ corresponds to a key depression depth obtained immediately after start of a let-off, and the second predetermined position P12′ corresponds to a key depression depth obtained immediately before the termination of the let-off. In this musical tone control system, a musical tone is controlled based on a key depression velocity calculated based on a time period from the turn-on of the first switch to the turn-on of the second switch.
As described above, in the conventional musical tone control system, the first switch and the second switch are configured to detect the key-depressed state of the key immediately after the start of the let-off and immediately before the termination of the same, respectively, i.e. both during occurrence of the let-off. However, during occurrence of the let-off, the motion of the key becomes unstable due to an increase in the rotational resistance of the key, and hence the time period from the turn-on of the first switch to the turn-on of the second switch can easily vary, which causes degradation of accuracy of calculation of the key depression velocity based on the time period. This problem becomes conspicuous particularly when a key depressing force is small, because the time period of occurrence of the let-off becomes longer and hence variation in the time period between the turn-on of the first switch and the turn-on of the second switch is liable to become larger.
The present invention has been made in order to solve the above problem, and an object thereof is to provide a musical tone control system for an electronic keyboard instrument, which is capable of calculating a key depression velocity of a key with high accuracy without being adversely affected by a large rotational resistance of the key during a let-off, and stopping sounding of a musical tone in appropriate timing.